In the field of semiconductor power electronics, endeavors are made to provide semiconductor devices with protection mechanisms in order that destruction of the semiconductor devices can be prevented even under extreme operating conditions.
Protection mechanisms of this type can be obtained in high-voltage freewheeling diodes by optimizing softness and robustness during a commutation operation. When an inductive load is turned off, chopping of a reverse current may occur, which may lead to overvoltage spikes and/or voltage oscillations that may be accompanied by destruction of the component. On the other hand, the reverse characteristic curve, that is to say the current/voltage characteristic curve of the freewheeling diode in reverse operation, may vary after hard commutation operations even though such a switching operation has not led to a hard failure of the component. Such variation of the reverse characteristic curve is manifested, for example, in an increase in the leakage current and is promoted for example by inhomogeneities in the rear side emitter and the rear side metal. Inhomogeneities of this type may occur in flat emitters produced by ion implantation, for example, by virtue of the fact that the area of the emitter is disturbed by small defects during the ion implantation or by spikes of a cathode metallization. In the case of implanted flat emitters, therefore, there are often inhomogeneities in their extent, whereby the emitter effect is attenuated.
The softness during the commutation operation was improved, for example, by introducing a more highly doped rear side emitter of the n type, but at the same time increased switching losses and possibly poorer switching robustness have to be accepted. Furthermore, proposals have been made for achieving a soft turn-off during the commutation of a semiconductor component such as a semiconductor diode or an IGBT by injection of charge carriers without fundamentally increasing the switching losses. In this case, a rear side emitter of the n conductivity type is preceded by so-called islands of the p conductivity type which together with the rear side emitter form a pn junction that breaks down before chopping of the diode reverse current occurs. As a result, the charge carriers generated via an avalanche mechanism continue to carry the load current and thus prevent the sudden current chopping, which leads to an alleviation of the harmful overvoltage spikes and/or oscillations. Via the hole injection, moreover, it is possible to prevent an electric field from building up at the nn+ rear side structure given by a drift area of the n conductivity type and the rear side emitter of the n+ conductivity type, whereby current chopping is likewise counteracted and the softness is thus improved.
A measure against characteristic curve rounding has been proposed by using stepped deep emitters. What is disadvantageous in that case, however, is that a measure taught therein cannot be combined with the abovementioned islands preceding the rear side emitter, since it is not possible to meet the boundary conditions in the dimensioning of the dopant areas of the emitter for setting the breakdown voltage of the pn junctions at the p-type islands.